There are hoisting machines, such as electric chain blocks and electric hoists, which use as a load hoisting motor an inverter-driven motor that is driven by an inverter incorporated in the hoisting machine main body. In such a hoisting machine, when the temperature of the inverter exceeds a predetermined set temperature, the inverter is tripped (shut off) to stop the operation of the hoisting machine from the viewpoint of safety. When a hoisting machine is operated at high frequency, i.e. when the operating time of the hoisting machine accounts for 60% or more of the sum total (100%) of the operating time and down time, a large amount of heat is generated from the inverter, and the heat undesirably stays in a control box housing the inverter. When the temperature in the control box exceeds the above-described predetermined set temperature (e.g. 100° C.), the inverter is tripped to stop the operation of the hoisting machine.
FIG. 1 is a sectional plan view showing the internal structure of a control box of a conventional electric chain block of the type described above. A control box 100 houses an inverter 102, an electromagnetic switch 103 and a transformer 104 that are mounted on a steel panel 101. When the electric chain block is operated at high frequency, a large amount of heat is generated from the inverter 102 as stated above. Because the panel 101 is made of steel, it is inferior in thermal conductivity to aluminum or other similar material. The panel 101 is also inferior in heat dissipation properties because it is thin in thickness. Therefore, the heat generated from the inverter 102 cannot escape but undesirably stays in the control box 100, causing the temperature to rise. As a result, the inverter 102 is tripped. It should be noted that reference numeral 105 in FIG. 1 denotes a speed reduction mechanism casing that houses a speed reduction mechanism (detailed later) of the electric chain block.
As a countermeasure against this problem, there is a method wherein, as shown in FIG. 2, the control box 100 is made of aluminum, and the inverter 102 is attached to the inner wall surface of the control box 100. With this method, the control box 100 is made of an aluminum material of good thermal conductivity, and the outer wall surface of the control box 100 is exposed to the surrounding air. Therefore, it is possible to expect that heat generated from the inverter 102 can be effectively dissipated. With this structure, however, wiring and maintenance are troublesome because the components other than the inverter 102, i.e. the electromagnetic switch 103 and the transformer 104, are attached to the main body side of the hoisting machine through the panel 101. Further, there is a fear that a possible impact on the control box 100 will be applied directly to the inverter 102.
Further, when the above-described hoisting machine lowers a lifted load, the load hoisting motor functions as a generator, and a regenerative electric current thus generated is passed through a regenerative braking resistor to consume it as heat, thereby regeneratively braking the load hoisting motor.
FIG. 3 is a diagram showing a structural example of a conventional regenerative braking resistor of the type described above. FIG. 3(a) is a plan view. FIG. 3(b) is a front view. FIG. 3(c) is a right-hand side view. As illustrated in the figures, a resistor 110 comprises a rectangular parallelepiped metallic casing 111 formed from a metal plate (e.g. an aluminum plate), resistance elements of continuous length (e.g. resistance elements each comprising a nichrome wire wound around a rod member of a heat-resisting insulating material) 112 disposed in the metallic casing 111, and a heat-resisting insulating material 113 of an inorganic material filled in the space in the metallic casing 111 other than the space occupied by the resistance elements 112. The resistance elements 112 are electrically connected in series at one end of each with a lead wire 115 in the metallic casing 111. Lead wires 114 connected to the other ends of the resistance elements 112 extend from an end of the metallic casing 111.
In the case of using as a regenerative braking resistor a resistor having resistance elements 112 disposed in a rectangular parallelepiped metallic casing 111 as stated above, when the hoisting machine is operated at high frequency, a large amount of electric current flows through the regenerative braking resistor, resulting in a rise in temperature. The temperature rise causes the temperature of the inverter to rise as well. If the inverter temperature exceeds the above-described set temperature, the inverter is tripped. In a case where the load hoisting motor generates a large regenerative electric current, a plurality of resistors 110 need to be used. In such a case, it takes time and labor to wire and install the resistors 110.
As a countermeasure to be taken under circumstances where the motor generates a large amount of regenerative electric current during lowering of a load, there is a method wherein, as shown in FIG. 4(a), an increased number of resistance elements 112 are disposed in the metallic casing 111. With this method, however, the resistance elements 112 radiate heat toward each other and thus release a large amount of heat. On the other hand, the surface area of the metallic casing 111 cannot be increased sufficiently. Thus, the method is inferior in heat dissipation properties. To cope with this problem, there has been proposed a method wherein, as shown in FIG. 4(b), the resistor 110 is equipped with a heatsink 120 as a discrete member. This method suffers, however, from the problem that there is an increase in the dimensions of the resistor 110 including the heatsink 120, particularly the height dimension. In addition, if the condition of contact between the heatsink 120 and the resistor 110 is not satisfactory, the heat dissipation properties degrade. Further, the use of the heatsink 120 increases the number of component parts correspondingly and causes an increase in cost.    Patent Literature 1: Japanese Patent Application Publication No. Hei 8-91784    Patent Literature 2: Japanese Examined Utility Model Application Publication No. Hei 5-39603    Patent Literature 3: Japanese Patent Application Publication No. Hei 10-32101